In recent years, it has become common practice to inject well stimulation fluids into selected oil- or gas-bearing geological formations or strata in order to increase the productivity of such formations. Such well stimulation fluids generally operate by hydraulic fracturing of and/or acidic reaction with the formation, increasing the permeability of the formation by oil or gas and the rate of delivery of oil or gas to the well-head. Hydraulic fracturing has long been practiced in the art using fluids such as native crude oil, lease crude oil, diesel fuel, kerosene, and water, with or without propping agents, such as sand, suspended therein (see, e.g., U.S. Pat. Nos. 3,070,165 and 3,089,846). Acidic treatment of a formation (frequently referred to as "acidizing") is also well-known in the art and is generally carried out using mineral acids such as HCl (in limestone formations) and HF (in sandstone formations), see, e.g., Williams, Gidley, and Schechter, Acidizing Fundamentals, SPE (1979), esp. pp. 1-18 and 29-37. It is also common practice to inject well cleanup fluids into oil- and gas-producing wells in order to increase the productivity of such wells. Such cleanup fluids generally remove scale and/or other foreign matter from the well bore. Such well cleanup fluids are well-known in the art, and are described in, e.g., Williams, Gidley, and Schechter, id., and in Allen and Roberts, Production Operations, Oil and Gas Consultants Intl. Inc. (1978), see esp. Vol. II, pages 118 and 171-180. For purposes of brevity, well stimulation and well cleanup fluids will sometimes be referred to hereafter as "well treatment fluids".
Some oil- and gas-bearing formations respond especially well to well treatment fluids containing methanol or aqueous methanol mixtures, particularly if such formations contain water-sensitive materials such as water-swellable clays. Such water-sensitive materials tend to interact with water present in the surrounding formation, and can swell and/or migrate within the formation, thereby reducing the permeability of the oil- or gas-bearing formation and diminishing the productivity thereof.
It is often useful to foam well treatment fluids prior to introduction of such fluids into the well bore. Such foamable fluids are described in, e.g., U.S. Pat. No. 3,937,283, and publications such as Blauer and Holcomb, "Foam Fracturing Shows Success in Oil and Gas Formations", Oil and Gas Journal (August, 1975) pp. 57-60, Blauer and Kohlhaas, "Formation Fracturing with Foam", paper (No. SPE 5003) presented at the fall meeting of the Society of Petroleum Engineers, Houston, Tex., Oct. 6-9, 1974, and Holcomb, "Foamed Acid as a Means of Providing Extended Retardation", paper (No. SPE 6376) presented at the 1977 Meeting of the Society of Petroleum Engineers, Conference on Permian Basin Oil and Gas Recovery, Midland, Tex., Mar. 10-11, 1977.
It is generally difficult to form stable, long-lasting foams in well treatment fluids containing appreciable amounts of methanol. Foamable, methanol-containing well treatment fluids currently used in the field are generally made foamable by adding thereto water and conventional hydrocarbon-based surface-active agents, such as sulfated polyethoxylated alcohols. It would be desirable to eliminate some or all of such water in well treatment fluids used in water-sensitive formations.
Certain substantially water-free methanol-containing foamable well stimulation fluids, containing fluorochemical-based surface-active agents, are described in Clark, Pike, and Rengel, "The Use of Fluorochemical Surfactants in Non-Aqueous Stimulation Fluids", paper (No. SPE 7894) presented at the 1979 meeting of the Society of Petroleum Engineers, International Symposium on Oil Field and Geothermal Chemistry, Houston, Tex., Jan. 22-24, 1979. Certain fluorochemicals for use as surface tension depressants in well stimulation fluids are described in Clark, Pike, and Rengel, "Water Soluble Fluorochemical Well Stimulation Additives", paper (No. SPE 9008) presented at the 1980 meeting of the Society of Petroleum Engineers, International Symposium on Oilfield and Geothermal Chemistry, Stanford, Calif., May 28-30, 1980, a paper relating to non-foamed aqueous well stimulation fluids. The former Clark, Pike, and Rengel paper No. SPE 7894 contains a disclosure of a substantially water-free methanol-containing well stimulation fluid, a fluid rendered foamable by use of a fluorochemical surface-active agent identified therein as "FC-B". The fluorochemical surface-active agent FC-B is a fluoroaliphatic radical-containing surface-active agent having ester linkages and polyoxyalkylene solubilizing moieties.
Foam-forming capabilities of a well treatment fluid can be demonstrated using a foam generator described in the Clark, Pike, and Rengel paper No. SPE 7894. Such foam-forming capabilities are expressed in terms of "foam quality" and "foam half-life". Foam quality represents the percentage of a foamed fluid which is occupied by trapped gas, with higher foam quality numbers representing more desirable foams. Foam quality is commonly denoted using the Greek symbol gamma (.GAMMA.). Foam half-life represents the time period in seconds required for one half of the liquid present in the foam to drain as a separate liquid phase, and is generally denoted by the symbol t.sub.1/2. Reagent grade methanol containing 1% of the above-described fluorine-containing oligomer was shown in the Clark, Pike, and Rengel paper No. SPE 7894 to have a foam quality of 60 and a foam half-life of 25 seconds.